In such industrial sectors as household electrical industries, manufacturing industries and automotive industries, motor drive units are used to provide rotating speed control, torque assist control, and positioning control of fans, pumps, compressors, conveyors, lifts, etc. In these sectors, small-sized highly efficient permanent magnetic motors (synchronous electric motors) are used extensively as the motor drive units. However, to drive the permanent magnet motor (called the PM motor hereunder) requires information about the magnetic pole positions of the motor rotor. This requirement makes position sensors such as resolvers and IC's indispensable. Recent years have seen widespread acceptance of so-called “sensorless control” schemes controlling the rotating speed and torque of PM motors without using position sensors.
Implementing sensorless control offers considerable benefits: it lowers the cost involved with position sensors (i.e., sensors themselves and their wiring). Also, with the sensors eliminated, the drive unit is made that much smaller and may be used in harsher conditions than before.
Today, the sensorless control of PM motors is implemented by adopting a number of techniques. One involves directly detecting the induced voltage generated by rotor revolutions (i.e., speed-induced voltage) and using the detected voltage as rotor position information for driving the PM motor. Also adopted is a position estimation technique for estimating by calculation the rotor positions based on a mathematical model of the motor of interest.
A big problem with the above-mentioned sensorless control techniques is how to detect positions at low-speed operation time. Most of the sensorless control schemes implemented today are based on the inducted voltage generated by the PM motor (speed-induced voltage). That means sensitivity is bound to drop in stopped or low-speed ranges where the induced voltage is low, leaving the position information buried in noise.
Meanwhile, there is a known position sensorless technique for use in low-speed ranges on the basis of 120-degree energizing control over the PM motor. This kind of technique can control the PM motor in speed ranges in which the induced voltage is low (e.g., see Patent Literature 1).